1. Field of the Invention
The present invention relates to a method and an apparatus for producing slabs in a continuous casting plant preferably equipped with a vertical mold, preferably for thin slab plants for casting preferably steel having, for example, a solidification thickness of 60 mm-120 mm, for example, 80 mm, and casting speeds of up to 10 m/min. and a maximum casting output of about 3 million tons per year.
2. Description of the Related Art
The thin slab plants known in the art for producing a slab thickness reduction, realized in a casting and rolling device, reduce the strand thickness immediately underneath the continuous casting mold, which is equipped with one or two pairs of foot rollers, predominantly in the so-called "segment 0". In that segment, the thickness of the strand is reduced, for example, from 65 mm to 40 mm over a metallurgical length of about 2 m, i.e., over the entire length of the segment or stand 0, which is not arranged vertically, wherein the casting speed is at most 6 m/min. A plant having these characteristics results in a strand thickness reduction of at most 38% and a deformation speed in the strand thickness of at most 1.25 mm/s.
During this holding time of the strand with liquid core, the strand shell having a thickness of about 8-12 mm is substantially deformed when entering the segment 0 due to bulging of the strand shell between the rollers of the continuous casting plant. This internal deformation increases with increasing casting speed and height of the plant or also the ferrostatic pressure, and decreases with decreasing spacing between the rollers. It is to be noted in this connection that the roller diameter cannot be less than, for example, 120 to 140 mm because of mechanical construction criteria, i.e., mechanical load, structural limits particularly in the case of intermediately arranged rollers. A possible mechanical solution could be a sliding plate, also called "grid", which, however, is not suitable for carrying out a reduction of the strand thickness.
In normal continuous casting, the internal deformation is essentially determined by
bulging of the strand between rollers; PA1 bending of the strand from the vertical into the inner circular arc; PA1 straightening of the strand into the horizontal; PA1 deviation of the rollers from the ideal strand guiding line due to PA1 roller jumps; PA1 roller impacts; and PA1 tensile stress. PA1 a minimum ferrostatic pressure or also a minimum plant height between the meniscus in an oscillating vertical mold, advantageously driven hydraulically, and the final solidification in the horizontally extending portion of the strand guiding means; PA1 minimized deformation density distribution of the total deformation composed of casting and rolling deformation and the bending deformation in a vertical bending unit with concavely constructed wide sides of the mold, predetermined roll diameters in the strand guiding means and up to maximum casting speeds of, advantageously 10 m/min; PA1 a complete elimination of the overheating phase or penetration zone for rising oxides in the vertical portion of the continuous casting plant, i.e., in segment 0 which is the machine element for carrying out the strand thickness reduction at a maximum casting speed of, for example, 10 m/min, for ensuring a strand symmetry in the range of overheating or pure molten steel phase; PA1 a casting and rolling process at maximum casting speed of, for example, 10 m/min in segment 0 in which the two phase area melt/crystal is present in the middle of the strand at the latest at the end of the segment 0 which carries out strand thickness reduction or casting and rolling; PA1 a deformation speed of the strand shell in segment 0 of at most 1.2 mm/s; PA1 a minimized bending deformation density in segment 1 from the vertical through several bending points into the inner circular arc independently of the casting and rolling deformation in the segment 0 which is arranged directly in front of segment 1; and PA1 a minimized straightening deformation density from the inner plant radius through several straightening or return bending points into the horizontal, preferably at least 12 s or at least 2 m in front of the final solidification in relation to an average casting speed of 80% of the maximum casting speed.
Added to these internal deformations and also the surface deformations must be the deformations which are produced by the strand thickness reduction or also the casting and rolling process in the segment 0. This specific internal deformation is superimposed on the deformation already produced in the segment 0 caused essentially by the strand bulging and the bending process from the vertical into the internal circular arc. This cumulation of the individual specific deformations may lead to a total deformation which becomes critical and leads to rupture not only of the inner strand shell but also the outer strand shell.
This type of additional load acting on the strand shell due to casting and rolling or the thickness reduction during the solidification in the segment 0 having a length of, for example, 2 m immediately underneath the mold is described in German patents 44 03 048 and 44 03 049 and is illustrated in detail as an example in the diagram of FIG. 1 of the drawing.
As shown in FIG. 4, a vertical mold having a length of 1 m and provided with one or two pairs of foot rollers is followed by a segment 0 having a length of 2 m in which the strand is bent over several stages into the inner circular arc and is also reduced in its thickness. These two processes or deformations taking place simultaneously lead to a superimposed cumulated total deformation composed of the bending deformation D-B and the casting and rolling deformation D-Gw. The total deformation D-Ge which acts on the strand shell may become greater than the critical limit deformation D-Kr and may lead to ruptures of the inner strand shell as well as of the outer strand shell. This danger increases with increasing casting speed due to a roller spacing or roller diameter in segment 0 which may not become smaller than a certain limit because of mechanical reasons.
In addition, when describing this problem, it must be taken into consideration that the limit deformation D-Kr has a specific behavior in each steel quality. For example, a deep drawing quality is less critical with respect to the absorption of deformations without the consequences of ruptures than, for example, a microalloyed steel quality API X 80.
Moreover, the configuration and extension of the overheated melt or also of the pure molten steel phase in the strand, indicated by the straight line G1 in dependence on the casting speed, has a significant influence of the internal quality of the strand. In the example illustrated in FIG. 1, the pure molten steel phase or also the geometrically lowest liquidus temperature in the middle of the strand extends up to about 1.5 m below the meniscus or casting level at a casting speed VG of 5 m/min and to about 3.0 m underneath the casting level at a casting speed VG of 10 m/min. Underneath this point, the two phase area composed of melt and crystal is present over the entire strand thickness, wherein the two phase area looses melt portion in favor of crystal portion proportionally with increasing distance in the direction toward the sump tip or the final solidification.
When the crystal portion is 50%, i.e., at half the distance between the lowest liquidus point of 1.5 m at, for example, VG5 m/min and the final solidification which takes place at about 15 m, i.e., at 8.25 m(1.5 m+(15 m-1.5 m).times.0.5=8.25 m) (percent by weight), the melt/crystal phase has a viscosity of 10,000 cP. When the crystal portion is 80%, the two phase area has a viscosity of 40,000 cP, while the pure molten steel phase, depending on the steel quality, has to the lowest liquidus point a viscosity of only about 1-5 cP and, moreover, its partial viscosity between the crystals (crystal network or dendrites) is practically not increased, i.e., is constant, up to the final solidification.
To provide a reference of the viscosities in the two phase area mentioned above to known substances of everyday life, the following substances shall be mentioned:
______________________________________ Water at 20.degree. C. 1 cp = 10 exp3 Ns/m exp2 Olive oil at 20.degree. C. 80 cp = Honey at 20.degree. C. 10000 cp Nivea at 20.degree. C. 40000 cp Margarine at 20.degree. C. 100000 cp Bitumen at 20.degree. C. 1000000 cp ______________________________________
These viscosities illustrate that for a good forced convection and, thus, a good destruction of crystals by a strand thickness reduction, a crystal/melt structure should be present in the core of the strand, i.e., at maximum casting speed the strand should have in its core already a two phase area in the region of the segment 0 or the pure molten steel phase or also the overheated area or the penetration zone for the rising of oxides should no longer be present. These conditions in connection with the oxidic degree of purity have led to the finding that, on the one hand, the segment 0 should be vertical and, on the other hand, the segment 0 should only serve for the strand thickness reduction and not also additionally for bending the strand.
In FIG. 1, which illustrates the poor conditions described above, the overheated zone or the lowest liquidus points extends to the end of the segment 0 and, thus, already into the inner circular arc of the continuous casting plant in the case of a maximum casting speed of 10 m/min, as indicated by point 1.1 on straight line G1. These casting conditions are extremely unfavorable for the strand shell deformation as well as for the oxidic degree of purity.
The two phase area, extending between two straight lines, i.e., the straight line G1 for the arrangement of the lowest liquidus point in dependence on the casting speed and the straight line G2 for the lowest solidus point or the final solidification in dependence on the casting speed, begins in the case of the maximum casting speed of 10 m/min at the end of segment 0 which carries out the strand thickness reduction.
In FIG. 3 of the drawing, partial illustration 3a, i.e., the left half of FIG. 3, also shows as an example the pattern of the different phases of a strand having a thickness of 100 mm from the meniscus in the mold with a subsequent strand thickness reduction in the segment 0 having a length of 2 m from 100 mm to 80 mm solidification thickness to the final solidification in the last segment number 14 for the maximum casting speed of 10 m/min. Partial illustration 3a once again makes it very clear that segment 0 imparts into the strand the highest possible deformation caused by the strand thickness reduction and the bending process from the vertical into the inner circular arc through five bending points as well as poor conditions for oxides rising into the meniscus, and, thus, into the casting slag.
Partial illustration 3a also illustrates that the reduction speed which acts on the shell of the strand for reducing the thickness from 100 mm to 80 mm, i.e., by 20%, is 0.833 mm/s at a casting speed of 5 m/min and is 1.66 mm/s at a casting speed of 10 m/min. This reduction speed of the strand thickness represents a direct measure of the deformation of the strand shell which at the entry into the segment 0 has a thickness of about 10.3 mm at a casting speed of 5 m/min and about 7.3 mm at a casting speed of 10 m/min. This strand deformation caused by casting and rolling is high and is not only doubled from 0.83 to 1.66 mm/s by the speed increase from 5 to 10/min, as expressed by the simplified variable 1.66 mm/s, but the speed increase enters the deformation with a quadratic function.
These high deformations, additionally superimposed by the bending processes in segment 0, lead to the danger of cracks of the inner strand shell as well as of the outer strand shell, particularly in the case of steel qualities which are sensitive to cracks.